Blackfly

Some black aphid species, particularly the black bean aphid, are troublesome on some vegetables and other plants during spring and summer.

Blackfly on broad bean

Blackfly on broad bean

Quick facts

Common name Blackfly including the black bean aphid
Scientific name Aphis fabae species group
Plants affected A very wide range including broad, French and runner beans, nasturtium, dahlia, philadelphus, viburnum and euonymus
Main cause A black sap-sucking insect
Timing April-August

What are blackfly?

There are several species of black aphid that can infest garden plants, a very common blackfly is the group of species known as the black bean aphid, Aphis fabae group. These is a sap-sucking insects can form dense colonies on the soft young growth of many plants.

Cherry trees are prone to a different aphid known as cherry blackfly.

Symptoms

The aphids are up to 2mm long and are mainly black but may have some white flecks on the upper surface of their bodies. Dense aggregations can rapidly develop on soft shoot tips, flower stems and on the underside of the younger leaves. The aphids are often attended by ants, which collect the sugary honeydew that aphids excrete the ants will also remove aphid predators such as ladybird larvae. Whitish cast skins of aphids often accumulate on infested plants.

Heavy infestations weaken the host plants and can result in stunted growth. On broad beans, pod formation can be poor if the plants become heavily infested. Flower formation on ornamental plants, such as dahlia, nasturtium and poppies, can be adversely affected when blackfly are feeding on the developing flowers. The winter-spring host plants, such as Philadelphus, Viburnum and common spindle (Euonymus europaeus) often develop curled foliage in response to the aphids feeding.

Control

Check susceptible plants frequently from spring onwards so action can be taken before a damaging infestation has developed.

Non-pesticide control

  • Where possible tolerate infestations of blackfly
  • Aphids have many natural enemies, including ladybirds, hoverfly larvae, lacewing larvae and several parasitic wasps. Some of these are available for biological control of aphids in greenhouses (aphid predators). In spring damaging aphid infestations often build  before natural enemies are active in sufficient numbers to give good control
  • Use finger and thumb to squash infestations on small plants
  • Picking off the tips of broad beans as soon as the blackfly are seen can reduce and delay infestation, and also improve the yield of beans

Pesticide control

  • Organic sprays, such as natural pyrethrum (e.g. Bug Clear Gun for Fruit & Veg, Ecofective Bug Killer), fatty acids (e.g. Solabiol Bug Free, Doff Greenfly & Blackfly Killer) or plant oils (e.g. Vitax Organic Pest & Disease Control, Bug Clear for Fruit and Veg) can give good control of aphids. These pesticides have a very short persistence and so may require reapplication to keep aphid numbers in check. Plant oil and fatty acid products are less likely to affect larger insects such as ladybird adults
  • More persistent insecticides include the synthetic pyrethroids lambda-cyhalothrin (e.g. Westland Resolva Pest Killer), deltamethrin (e.g. Provanto Ultimate Fruit & Vegetable Bug Killer) and cypermethrin (e.g. Py Bug Killer)
  • The systemic neonicotinoid insecticide acetamiprid (e.g. Bug Clear Ultra) is also available
  • Follow label instructions when using pesticides. On edible plants make sure the food plant is listed on the label and follow instructions on maximum number applications, spray interval and harvest interval
  • Plants in flower should not be sprayed due to the danger to bees and other pollinating insects
  • Inclusion of a pesticide product does not indicate a recommendation or endorsement by the RHS. It is a list of products currently available to the home gardener

Downloads

Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)

Biological control suppliers (Adobe Acrobat pdf document)

Biology

Blackfly Aphis fabae overwinters as eggs on shrubs such as common spindle (Euonymus europaeus), Viburnum and Philadelphus. Egg hatch occurs in spring as the winter host plant is coming into new leaf. Several generations of wingless aphids, which are all female and produce live young rather than eggs, develop on the spring foliage. By May, the foliage on the winter host plants has become older and tougher, which along with increasing day length induces a change in the aphids. A generation of female winged aphids develops that fly away in search of suitable summer host plants, such as beans, nasturtium, poppies and dahlia. At that time of year male aphids are not required for reproduction, so a single winged aphid arriving on a broad bean can quickly establish a new colony. Blackfly infestations on the winter-spring host plants die out during May-June.

At the end of summer, another winged generation of aphids is produced that migrates back to the winter host plants. At that time there are males and females that will mate before the females deposit eggs around buds and in crevices on the stems.


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